The invention relates generally to gas producing compositions and more particularly to double base propellants having a plateau pressure-burning rate relationship.
Solid propellants are often classified as being either homogeneous or composite. The former refers to those types, which are considered true monopropellants in which each molecule contains all the necessary fuel and oxygen for combustion. The composite type propellant, in contrast, consists of a physical mixture of a fuel and an oxidizer. The homogeneous propellants are further subclassified as being either single, or double base, depending on whether the composition contains a single energetic combustible or contains an additional energetic combustible or a mixture of additional energetic combustibles which acts as an energetic plasticizer for the first energetic combustible.
The burning rate equation for double base propellants is given as r= KP.sup.n or log r= n log P+ log K where r is the burning rate, P is the combustion chamber pressure, K is a constant for each propellant composition, and n is a constant for non-modified propellants but is a variable function in plateau propellants varying from very high positive values through zero to low negative values. Thus, a plot of log r against log P would give a straight line with a slope of n for a non-modified propellant, but a "plateau" shaped line for plateau propellants. Plateau propellants are also frequently referred to as modified propellants. In this patent application modified double base propellant means the same as plateau double base propellant.
The plateau logarithmic relationship between the burning rate and the chamber pressure is greatly preferred over the linear logarithmic one. Such a relationship gives better ballistic and combustion stability, less dependence on initial temperature, and lower peak pressures in the combustion chamber.
In order to obtain the plateau relationship, additives referred to as ballistic modifiers are included in the propellant composition. These additives accelerate the burning rate at low pressures but have a decreasing catalytic effect as the chamber pressure increases up to a certain pressure. As the catalytic effect diminishes, the rate-pressure relationship slowly approaches the rate-pressure relationship for the propellant without the modifier. Sometimes the rate-pressure function for the plateau propellant actually drops below that of the unmodified propellant.
The location of the "plateau" on a log-log graph of the pressure-burning rate relationship is important for certain applications. For example, it is often desirable to have the "plateau" phenomena to occur as soon as possible in propellants for auxiliary gas generating equipment. It is also desirable to be able to adjust the plateau rate level vertically.
Certain control over the location of the plateau can be achieved by carefully formulating the propellant or selecting a particular ballistic modifier. Such changes often cause new problems which would not have occurred if the placing of the "plateau" was achieved by the addition of another ingredient.
However desirable the plateau pressure-burning rate relationship may be, it is often sacrificed in order to obtain other propellant properties. Two such properties are high volume of, and nitrogen content in, the exhaust gas of the propellant combustion.
Nitrogen is considered the ideal exhaust or working gas because it is smokeless, does not flash, does not dissociate, is not toxic, and has a favorable molecular weight. The number of moles of exhaust gas is inversely proportional to the average molecular weight of the exhaust gas. Hence a large proportion of nitrogen in the exhaust gas means a large volume for the exhaust gas. These properties of nitrogen are especially important for auxiliary gas producing equipment.